This invention relates to a method for preparing low molecular weight branched siloxanes, especially methyltris(trimethylsiloxy)silane, useful as industrial siloxane lubricants, cosmetic fluids and cleaning agents.
Heretofore, low molecular weight branched siloxanes as represented by the following general formula (3): 
wherein R is hydrogen or a monovalent hydrocarbon group having 1 to 20 carbon atoms have scarcely been utilized because simple and convenient methods have not been available for their synthesis. This is because, as compared with low molecular weight linear siloxanes, intramolecular branching causes gelation or formation of more by-products during usual equilibration or polymerization reaction. It is very difficult to synthesize a desired low molecular weight branched siloxane in an efficient manner.
An object of the invention is to provide an efficient method for preparing low molecular weight branched siloxanes of formula (3), especially methyltristrimethylsiloxysilane, in high yields.
It has been found that a low molecular weight branched siloxane of the general formula (3) is efficiently prepared by reacting a trichlorosilane of the general formula (1) with a disiloxane of the general formula (2) in the presence of a linear phosphonitrilic chloride (LPNC) catalyst.
RSiCl3xe2x80x83xe2x80x83(1) 
Herein R is hydrogen or a monovalent hydrocarbon group having 1 to 20 carbon atoms.
According to the invention, a low molecular weight branched siloxane is efficiently prepared by reacting a trichlorosilane with a disiloxane in the presence of a LPNC catalyst.
The trichlorosilane and disiloxane used herein as the starting reactants have the general formulae (1) and (2), respectively. 
Herein, R is hydrogen or a monovalent hydrocarbon group having 1 to 20 carbon atoms, examples of which include alkyl groups such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, dodecyl and octadecyl, cycloalkyl groups such as cyclohexyl, alkenyl groups such as vinyl and allyl, aryl groups such as phenyl and tolyl, aralkyl groups such as benzyl, and substituted groups in which some or all of the hydrogen atoms on the foregoing groups are substituted with fluorine. A plurality of R""s may be identical or different. Of these, methyl is most preferred for ease of reaction.
Methyltrichlorosilane is most preferred among the trichlorosilanes of formula (1), and hexamethyldisiloxane is most preferred among the disiloxanes of formula (2).
The trichlorosilane and disiloxane are preferably used in a molar ratio between 1:1 and 1:9. To increase the amount of a desired low molecular weight branched siloxane formed and suppress the formation of by-products, use of the disiloxane in excess relative to the trichlorosilane on a molar basis is recommended. Most preferably, the trichlorosilane and disiloxane are used in a molar ratio of 1:4.5.
The low molecular weight branched siloxane produced by reacting the trichlorosilane of formula (1) with the disiloxane of formula (2) in the presence of a LPNC catalyst has the general formula (3) wherein R is as defined above. 
Particularly when methyltrichlorosilane as the trichlorosilane of formula (1) and hexamethyldisiloxane as the disiloxane of formula (2) are reacted in a molar ratio in the above range in the presence of a LPNC catalyst, a low molecular weight branched siloxane of the following general formula (4) is obtainable in high yields in the form of a reaction solution which is easy to purify by distillation in the subsequent step and contains less by-products. 
Especially when methyltrichlorosilane and hexamethyldisiloxane are reacted in a molar ratio between 1:1 and 1:9, desirably 1:4 and 1:6, in the presence of a LPNC catalyst, methyltris(trimethylsiloxy)silane is selectively synthesized.
The catalyst used herein is a linear phosphonitrilic chloride (LPNC) catalyst. The synthesis of this catalyst is known in the art as disclosed in J. Emsley et al., J. Chem. Soc. A (1971) 2863. See also U.S. Pat. No. 3,839,388, German Patent OS 2229514, JP-A 11-267508 and many other patent publications. The reaction promoting ability of a particular LPNC catalyst largely depends on the amounts of phosphorus and nitrogen used in the synthesis of that catalyst. Preferably, the molar ratio of phosphorus to nitrogen is from 1:1 to 4:1, more preferably from 2:1 to 3:1.
It is noted that most commercially available cyclic phosphonitrilic chlorides (CPNCs) are little active.
It is also noted that in JP-B 45-41599, organic silicon halide compounds are synthesized by carrying out substitution reaction, similar to the present reaction, in the presence of a LPNC catalyst. This patent, however, fails to efficiently synthesize low molecular weight branched siloxanes desired in the present invention and does not refer to the activity of LPNC catalyst used.
The LPNC catalyst is used in a catalytic amount. Too small an amount of the LPNC catalyst may retard the reaction whereas with too large an amount, the reaction may become too fast to control. Then the LPNC catalyst is preferably used in an amount of 0.01 to 1%, and more preferably 0.05 to 0.2% by weight based on the starting silane and siloxane combined.
The reaction temperature is preferably set in the range of 0xc2x0 C. to 100xc2x0 C. At temperatures below 0xc2x0 C., the reaction rate may become so slow that a long time is taken until the completion of reaction. The preferred temperature is from 15xc2x0 C. to 30xc2x0 C. In this case, the reaction time is about 1 to 6 hours.
After the completion of reaction, the end low molecular weight branched siloxanes are collected by conventional distillation of the reaction solution. If necessary, the LPNC catalyst is deactivated by any well-known technique, with better results being obtained.